This question asks about the color change of 7-hydroxyphenoxazone, the active compound in litmus paper, and the relation of that color change to the conjugated system in the molecule.


In Klaus Warzecha's answer, he mentions 4-nitrophenol as another, simpler indicating system, analogous to 7-hydroxyphenoxazone.


In both of these species, as noted by Klaus, deprotonation of the hydroxyl group to an -olate moiety enables a resonance between the substituents, through the molecular cores, which is not possible in the protonated species:

phenoxazole resonance

nitrophenol resonance

I'm interested in knowing whether this property of an "opened-up resonance on deprotonation" and the concomitant influence on the excitation spectrum are necessary characteristics for indicating compounds.

So: Are there any known examples of acid-base indicators (e.g., compounds exhibiting a change in visible-spectrum absorbance upon deprotonation) where the removal of the proton does not "open up" additional resonance structures in this fashion?

Clicking through some of the links on Wikipedia's $\mathrm{pH}$ indicators page, I would guess that cresol red, bromocresol green, and bromophenol blue potentially fall into this category, but I'm not confident enough in my evaluation of resonance structures to be certain.


In the examples you drew, these resonance structures are not ‘opened up’; they are there in both the protonated and the deprotonated form. However, since the protonated form requires charge separation to access these resonance structures, they are less important.

It rarely makes sense to do resonance structure bookkeeping like you are trying to implement in your question.

Indicators need to display a colour change upon protonation/deprotonation. that can either happen by modifying the structure (e.g. bromocresol green) or by the electronic changes induced by the charge occurring. The latter case will mean some shifting in the orbital energies — most importantly in HOMO and LUMO — and can practically always be explained in a ‘resonance way’.

  • $\begingroup$ In the latter case, electronic changes induced by the charge occuring, you say the change in HOMO-LUMO transition can be explained in "a resonance way". What do you mean? I have problems explaining the colour change when there is not a blatant change in extent of conjugation upon deprotonation. $\endgroup$
    – Adroit
    Dec 11 '16 at 9:19
  • $\begingroup$ @Adroit Best ask a question. $\endgroup$
    – Jan
    Dec 11 '16 at 12:35
  • $\begingroup$ chemistry.stackexchange.com/questions/61740/… $\endgroup$
    – Adroit
    Dec 11 '16 at 13:19

In contrast to 4-nitrophenol, cresol red, bromocresol green, and bromophenol undergo structural changes upon deportonation because the sultone opens up.

But again, you can draw quinoid resonance structures for the resulting anions.


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